The present invention relates to a microactuator and a method of forming the same, and more particularly to a micro-actuator suitable for driving a small size device such as optical devices, optomagnetic and magnetic devices.
A conventional small actuator is mounted on a top of a suspension of a magnetic head for driving a slider. This was proposed by L. S. Fan and is disclosed in IEEE Transactions On Industrial Electronics, vol. 42, No. 3, pp. 222-233, June 1995 entitled "Magnetic Recording Head Positioning At Very High Track Densities Using A Microactuator Based Two-Stage Servo System". FIG. 1 is a plan view illustrative of a conventional microactuator. FIG. 2 is a partially enlarged view of an area "A" in FIG. 1 illustrative of the conventional microactuator. FIG. 3 is a cross sectional elevation view illustrative of the conventional microactuator taken along an A--A line in FIG. 2. In FIGS. 1 and 2, illustrations of a structure of a platform are eliminated. The microactuator has a pair of stators 83 and 84 provided on opposite sides of a silicon substrate 100 and separated from each other in a first direction, and a movable part 82 positioned between the stators 83 and 84. The movable part 82 is supported by sprig members 81 which are provided on spring fixing stages 80 fixed to the silicon substrate 100 so that the movable part 82 is floated or isolated from tile silicon substrate 100.
Each of the stators 83 and 84 has a stator extending portion which extends toward inside regions and in the first direction and also extends along a longitudinal center line. The stator extending portion has many stator comb-tooths portions 91 which extend from both sides of each of the stator extending portions in a second direction perpendicular to the first direction, thereby to form a comb-shape, wherein the stator comb-tooth portions 91 are arranged at a first constant pitch in the first direction and extend in the second direction. Each of the stator comb-tooth portions 91 has comb-tooth shaped stator electrodes 93 which extend from one side of the stator comb-tooth portion 91, wherein the comb-tooth shaped stator electrodes 93 extend in the first direction at a second constant pitch.
The movable part 82 comprises first and second side frame portions extending in the first direction and separated form each other in the second direction and a center frame portion extending in the second direction to connect the first and second side frame portions to each other. Each of the first and second side frame portions has many movable comb-tooth portions 92 which extend from the side toward the longitudinal center line in the second direction, thereby to form a comb-shape, wherein the movable comb-tooth portions 92 are arranged at a third constant pitch in the second direction and extend in the first direction. The many movable comb-tooth portions 92 and the many stator comb-tooth portions 91 are alternately aligned in the first direction, whereby each of the many movable comb-tooth portions 92 is positioned between adjacent two of the many stator comb-tooth portions 91. Each of the movable comb-tooth portions 92 has comb-tooth shaped movable electrodes 94 which extend from one side of the movable comb-tooth portion 92, wherein the comb-tooth shaped movable electrodes 94 extend in the first direction at a fourth constant pitch, so that the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 are alternately aligned in the second direction, whereby each of the comb-tooth shaped movable electrodes 94 is positioned between adjacent two of the comb-tooth shaped stator electrodes 93. The stator comb-tooth portion 91 is wider in width than the movable comb-tooth portion 92. The comb-tooth shaped stator electrodes 93 are wider in width than the comb-tooth shaped movable electrodes 94. The comb-tooth shaped stator electrodes 93 are adhered onto the silicon substrate 100 together with the many stator comb-tooth portions 91. The comb-tooth shaped movable electrodes 94 are separated or floated from the silicon substrate 100 together with the movable comb-tooth portion 92.
A voltage is applied across the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 so that the movable part 82 is driven to be moved in the first direction. The voltage application across the comb-tooth shaped movable electrodes 94 of the movable part 82 and the comb-tooth shaped stator electrodes 93 of the second stator 84 causes the movable part 82 to move toward the second stator 84. The voltage application across the comb-tooth shaped movable electrodes 94 of the movable part 82 and the comb-tooth shaped stator electrodes 93 of the first stator 83 causes the movable part 82 to move toward the first stator 83.
As illustrated in FIG. 3, a platform 72 is provided over the movable comb-tooth portions 92 of the movable part 82, wherein the platform 72 has contact ridge portions 73 which are in contact with the top surfaces of the movable comb-tooth portions 92 of the movable part 82. In Fan et al. proposed structure, the contact ridge portions 73 are formed almost entirely over the movable comb-tooth portions 92 of the movable part 82. The platform 72 is used for allowing a magnetic head or a slider to be mounted thereon.
The above conventional microactuator has been fabricated as follows. A phospho silicate glass pattern of 2 micrometers in thickness is formed on a first region of the silicon substrate 100, wherein the first region is for later formation of the above movable part 82. A photo-resist pattern is then formed on the phospho silicate glass pattern by use of a photo-lithography technique. A copper plating method is carried out to form copper films between apertures of the photo-resist pattern. A platform pattern is then formed by use of other photo-lithography technique and subsequent copper plating method before the phosphosilicate glass pattern is removed by an etchant of hydrofluoric acid solution so as to isolate the movable part 82 and the comb-tooth shaped movable electrodes 94 from the silicon substrate 100, whereby the microactuator and the platform are formed. Namely, this actuator is formed by the electro-plating technique.
It has been known that a polysilicon thin film is used for the microactuator fabricated by utilizing a silicon IC process. This microactuator having the polysilicon thin film is superior in conformability with the silicon IC process and also in mechanical characteristics as compared to the above electroplated microactuator. In order to apply this second microactuator having the polysilicon film to the magnetic head or the optomagnetic head, it is, however, necessary to suppress motion of the head in other directions to the intended direction. For example, the microactuator shown in FIG. 1 is required to cause the movable part 82 to move in the first direction but required to suppress any motion of the movable part 82 in a vertical direction to the first and second directions. In order to suppress the motion of the movable part 82 in the vertical direction, it is effective to increase the thickness of the springs 81. In view of utilizing a large electrostatic force, it is also important to increase the thicknesses of the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93.
Accordingly, it had been required to form the actuator having a thickness of not less than 20 micrometers. Notwithstanding, a practically possible maximum thickness of the polysilicon film is about 4 micrometers which is much thinner than the required thickness. For those reasons, the above plating technique for forming the electro-plated actuator and the following other type microactuator had been developed. The other type microactuator may be formed by etching technique for etching a single crystal silicon layer.
The actuator having the silicon crystal silicon film may be formed by use of an silicon-on-insulator substrate. This technique was proposed by A. Benitez et al. and is disclosed in Sensors and Actuators, A50, pp. 99-103, 1995, entitled "Bulk silicon microelectromechanical devices fabricated from commercial bonded and etched-back silicon-on-insulator substrates". The use of this technique allows the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 to be formed from a single crystal silicon layer of 20 micrometers in thickness. The comb-tooth shaped stator electrodes 93 and the many stator comb-tooth portions 91 are fixed through a silicon oxide layer to the silicon substrate 100. The silicon oxide film underlying the comb-tooth shaped movable electrodes 94 is removed by a hydrofluoric acid solution as an etchant, so that the comb-tooth shaped movable electrodes 94 is made floated or isolated from the silicon substrate 100. Since the comb-tooth shaped movable electrodes 94 is narrower in width than the comb-tooth shaped stator electrodes 93 and also the movable comb-tooth portions 92 is narrower in width than the stator comb-tooth portions 91, the silicon oxide film remains under the comb-tooth shaped stator electrodes 93 and the stator comb-tooth portions 91 even after the wet etching has been carried out by use of the hydrofluoric acid solution. The comb-tooth shaped movable electrodes 94 having the single crystal silicon film and the comb-tooth shaped stator electrodes 93 also having the single crystal silicon film may be formed over the silicon substrate 100.
The above conventional fabrication method is, however, engaged with the following four problems.
The first problem is concerned with a difficulty for the electroplating actuator to comply with the requirement for high speed responsibility of the microactuator. In the electroplating technique, any available materials for the platform are limited into metals. The metals have relatively large densities. For example, copper is 8.9. The use of the metal platform cases the platform weight to become heavy. The combination of the platform and the movable part forms a resonator of the microactuator, for which reason the increase in the weight of the platform causes a resonant frequency to be dropped. The drop in resonant frequency of the microactuator causes a response speed of the mnicroactuator to become slow, even a high speed responsibility is actually required for applying the microactuator to the advanced magnetic head or optomagnetic head.
The second problem is concerned a complication in fabrication process for the microactuator. A voltage is applied across the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 in order to drive the microactuator As illustrated in FIG. 3, the platform 72 is equal in potential to the movable comb-tooth portions 92 of the movable part 82, for which reason a voltage application across the movable comb-tooth portions 92 and the stator comb-tooth portions 91 causes an electrostatic force to be applied to the platform 72, whereby the platform 72 is attracted toward the stator comb-tooth portions 91. Since the stator comb-tooth portions 91 is positioned under the platform 72, the platform 72 is thus forced by the electrostatic force to move downwardly. A displacement of the platform 72 downwardly by the electrostatic force depends upon a level of the voltage applied across the movable comb-tooth portions 92 and the stator comb-tooth portions 91. This displacement in the vertical direction or the downward direction of the platform 72 makes it difficult to realize a highly accurate positioning of the head. In order to suppress the displacement in the downward direction of the platform 72 by the electrostatic force depending upon the applied voltage level, it is effective to increase a distance between the platform 72 and the stator comb-tooth portions 91. In order to increase the distance between the platform 72 and the stator comb-tooth portions 91, it is required to increase the height of the contact ridge portions 73 of the platform 72. This increase in the height of the contact ridge portions 73 of the platform 72 causes the fabrication process to be complicated.
The third problem is concerned with a low yield of the microactuator due to complicated and many fabrication processes therefor. The actuator having the platform is fabricated by the sequential and complicated processes as described above. A large number of the fabrication processes and the complication of the processes make it difficult to increase the yield of the microactuator. Particularly, it is difficult to form the platform after the microactuator has been fabricated because it is necessary to form the contact ridge portions 73 and the platform 72 over the microactuator pattern having a large unevenness of more than 20 micrometers. Further, the satisfaction to the requirement for increase in the height of the contact ridge portions 73 of the platform 72 makes it difficult to form the microactuator in the sequential fabrication processes.
The fourth problem is concerned with no concrete proposal having been made for how to mount the platform onto the movable part in the microactuator using the single crystal silicon layer. It may be considered to form the platform by use of the electroplating technique on the movable part of the microactuator having already been formed. However, the use of the electroplating technique for forming the platform on the movable part of the microactuator causes the above first, second and third problems as described above.
In the above circumstances, it had been required to develop a novel microactuator and a method of forming the same free from the above first, second, third and fourth problems.